1. Field of the Invention
This invention relates to phosphors capable of emitting radiation at high efficiency when subjected to ultraviolet radiation, X-rays, electron bombardment or some other forms of excitation, and particularly to phosphors useful as luminescent materials, illuminating materials, photosensitive materials, display materials, sensitizing materials, photographic materials, image-resolving materials and basic materials for optoelectronics field.
2. Description of the Prior Art
Phosphor compositions activated with rare-earth elements have already been put to practical use as red-dot phosphors in color television cathode-ray tubes or color kinescopes. Substantially all of the phosphor compositions which have been put to practical use are inorganic compounds. As an example of such inorganic compounds, an inorganic compound typified by the compound of the formula (Y.sub.1-x Eu.sub.x).sub.2 O.sub.2 S which is disclosed in Japanese Patent Publication No. 13242/1972 may be cited. The aforementioned (Y.sub.1-x Eu.sub.x).sub.2 O.sub.2 S is the compound of the ceramic matrix Y.sub.2 O.sub.2 S (yttrium oxysulfide) doped with the activator Eu (europium), one of the rare-earth elements. Since the phosphors activated with rare-earth elements produce characteristic narrow and sharp line emission spectra, they are drawing amounting attention as basic materials, for example, for application to the field of color television systems and lasers which are expected to provide chromaticities of extremely high levels of purity, namely the field of high-function phosphor. The ceramic phosphors activated with transition metal elements including rare-earth elements such as, for example, europium, have a disadvantage that they do not produce effective luminescence unless the optimum matrixes comprising a proper metal element capable of substituting the metal element used as the activator and an inorganic substance used as the matrix including the proper metal element (corresponding to the yttrium oxysulfide disclosed in Japanese Patent Publication No. 13242/1972) are selected quite appropriately. No fundamental knowledge has yet been acquired concerning the interrelations between the activating elements, matrixes and luminescent efficiencies. Worse still, in the synthesis of practically satisfactory ceramic phosphor compositions, the selection of combinations between their component elements and activating elements is extremely difficult for the reason indicated above and the process for their manufacture has a fatal drawback that, because of the complication of procedure involving a pretreatment, calcination at elevated temperatures, annealing, several cycles of sintering and aftertreatment, it hardly befits mass production. Moreover, the operation for transparentization necessitates an elaborate and complicate technique and the equipment and conditions for the production themselves are difficult of effective application to large products and the costs of production are extremely high. Thus, a number of drawbacks are suffered also in terms of manufacturing process and product quality. Further in terms of application, independent use of ceramic phosphors is restricted. Such phosphors prove useful in rigidly limited forms for virtually all of them acquire their significance only in the presence of binders such as of plastic materials. Naturally, it is impracticable for the phosphors to be effectively used in liquid state in which they are dissolved in certain kinds of organic solvents.
With a view to overcoming the various drawbacks mentioned above, extensive studies have been conducted in the phase of organics, namely on the phosphors of organic rare-earth compound typified by rare-earth metal chelates, incorporating .beta.-diketones [e.g., K. C. Joshi et al, Journal of Inorganic and Nuclear Chemistry, 35 (9) 3161, (1973)] as the ligand, for example. As the result, some of the phosphors have been demonstrated to possess a potentiality of laser oscillation and exhibit luminescence at high efficiency. The methods for their production, however, are very complicate and hardly feasible and are scarcely fit to mass production. For practical use as phosphors, the products by these methods suffer from fatal chemical and physical drawbacks such as excessively inferior thermal stability and ready degradation of the luminescent ability due to chemical decomposition with lapse of time, for example.
Another approach to the problem has been made through studies on systems using organic carboxyl groups. Examples of the studies reported in literature are:
(1) V. F. Zolin et al., Zh. Prikl. Spektrosk., 17 (1) 71 (1972) PA1 (2) V. F. Zolin et al., Optics and Spectroscopy, 33 (5) 509 (1972) PA1 (3) N. A. Kazanskaya et al., Optics and Spectroscopy, 28 (6) 619 (1970) PA1 (4) V. L. Ermolaev et al., Optics and Spectroscopy, 28 (1) 113 (1970) PA1 (5) "Sinha, S. P.: Z. Naturforsch, 20a 319 (1965)" cited in Molecular Crystals, 1 37 (1966) PA1 (6) H. G. Brittain, Inorganic Chemistry, 17 2762 (1978)
As dealt with in these articles of literature, the systems in question are chelate-like compounds similar to the aforementioned .beta.-diketone chelates. These compounds have a disadvantage that they possess poor thermal stability and suffer from degradation of their luminescent through a change on standing. Substantially all of these compounds are soluble in water. The reports treat of the transfer of energy in aqueous solutions and the phenomenon of luminescence occurring in consequence of the energy transfer.